Electronic variable message signs are used in transport applications to give route information to motorists. The signs have electro-optical characters which are selectively illuminated to display a message to passing motorists.
The optical performance of the electro-optical characters have to meet the stringent requirements of the Highways Agency. To meet these requirements existing signs use clusters of light emitting diodes which are driven hard to achieve acceptable brightness. Each cluster of light emitting diodes is shielded to give good visibility in the brightest ambient conditions.
Conventional light emitting diodes are encapsulated within a cylindrical clear resin capsule which has a domed end. Light rays from the light emitting diode are collected and directed by the domed end into a cone of light which is either symmetrical or asymmetrical around the principal axis of the cylinder. More than 80% of the generated light is lost by being emitted and subsequently scattered outside the primary viewing cone.
British patent application number GB 2,282,700-A describes an optical element which is used in conjunction with conventional light emitting diodes to redirect the light scattered outside the primary viewing cone. The optical element thereby increases the amount of useful light emitted by each of the light emitting diodes.